The invention relates generally to a bias level generating circuit in a flash memory device for generating bias levels suitable for a programming or a programming verification of a cell.
Referring now to FIG. 1, a bias level generating circuit in the conventional flash memory device will be explained below. The bias level generating circuit is consisted of a first inverter I1 for inverting an enable signal EN, a first switching element P1 for transferring the power supply to a charge pump 10 according to the output signal from the first inverter I1, a charge pump 10, a voltage dividing diode, a regulation sense amplifier 20, and switching elements and resistors for forming a current path.
The charge pump 10 generates a pump-out voltage PUMP_OUT according to the enable signal EN and the power supply. The voltage dividing diode is consisted of a plurality of diodes connected serially, for falling the pump-out voltage PUMP_OUT. The regulation sense amplifier 20 senses the signal the voltage of which is fallen by the plurality of diodes to generate a current path enable signal, according to the regulation enable signal REG_EN. The second switching element NI forms the current path to the output terminal of the charge pump 10 along with the first resistor R1, according to the output signal from the regulation sense amplifier 20.
Below, the operation of the circuit will be explained.
If the enable signal EN is applied with a High state, the first inverter I1 inverts the enable signal EN to turn on the first switching element P1. Then, the first switching element P1 turned on transfers the power supply to the charge pump 10. The voltage of the output signal from the charge pump has fallen by the plurality of diodes D1-D4 and is applied to the regulation sense amplifier. Then, the regulation sense amplifier 20 driven according to the regulation enable signal REG_EN senses the output signal of the charge pump 10 the voltage of which has fallen, thus turning on the second switching element N1. Next, a current path extending from the pump-out voltage PUMP_OUT of the charge pump 10 to the first resistor R1 is formed by the first switching element N1, so that the pump-out voltage is controlled to a bias level for use in a programming or a programming verification of the cell.
As mentioned above, in the conventional charge pump and regulation circuit structure, a single charge pump and a single regulator generate different biases levels necessary for a programming or a programming verification of a cell. Thus, the regulator having this structure has only a single current path (discharge path) and therefore the size of the transistor for current path must be fitted to a high bias. Therefore, in case that a low bias is generated using this circuit structure, there is a problem that it consumes a lot of time to discharge the current using a transistor for use in a high bias current path, in order to adjust the current to a desired level. Also, there is a problem that oscillation in the bias level is severe because a great current has to be discharged at a time. This oscillation causes malfunction in the programming verification of a flash memory cell.
It is therefore an object of the present invention to provide a bias level generating circuit in a flash memory device capable of preventing oscillations, by which a bias level suitable for a corresponding operation is generated, when a programming operation or a programming verification operation is performed.
In order to accomplish the above object, a bias level generating circuit in a flash memory device according to the present invention is characterized in that it comprises a first inverter for inverting an enable signal, a first switching element switched depending on the output signal from the first inverter, a charge pump for generating a pump-out voltage necessary when generating a bias level depending on the power supply send via the first switching element and the enable signal, a bias switch circuit for preventing oscillation of the pump-out voltage in order to generate a bias level necessary for a programming or a programming verification of a cell, depending on a program signal, and a regulator for making the voltage generated in the bias switch circuit being a bias level suitable for the programming or the programming verification of the cell, by setting a specific path current depending on the program signal.